Frederick II of Hohenstaufen, Holy Roman Emperor, King of Sicily, King of Jerusalem, has, since his death in 1250, enjoyed a reputation as one of the most remarkable monarchs in the history of Europe. His wide cultural tastes, his apparent tolerance of Jews and Muslims, his defiance of the papacy, and his supposed aim of creating a new, secular world order make him a figure especially attractive to contemporary historians. But as David Abulafia shows...